Shadowed variables in inner classes

Yesterday we had a bug caused by the following situation: a class X had a variable “user”. Class X had an inner, anonymous class extending class Y. Inside the inner class a reference was made to the “user” variable from the enclosing class X. Somebody then added a protected variable “user” to class Y. The java compiler silently updated the reference to “user” within the inner class from X.user to Y.user.

This is pretty nasty and I hoped that some of the code-checking tools would warn about about it, so I checked the code with PMD, Findbugs and Checkstyle. Unfortunately, none of them flagged the issue.